Particulates also come from the abrasive friction of tyres on the road and brakes being worn to dust.

This means electric vehicles will help tackle pollution, but won’t solve it.

Then there’s the complexity of the pollutants themselves. Prof Lewis says a single pollution particle can contain 100,000 chemicals.

Diesel alone, he says, contains 25,000 chemicals.

“The composition of particles is wildly complicated.

When we are monitoring emissions we just weigh them!

We can’t attempt to discern which chemicals are most problematic.

“Even so, what we know about particles is way ahead of how they are regulated.”

Prof Lewis suggested that the government might choose to submit new data into their long – and so far unsuccessful – court battle with the pressure group ClientEarth over the failure to meet NO2 targets.

ClientEarth’s lawyer Alan Andrews told BBC News:

“This study shows that we need more evidence to know exactly how much older cars are polluting. We strongly support that.

“It not only highlights a gap in the evidence but also how the government has failed to get a grip on the problem over the years.

“To lower ambition now would be hoping for the best and have real implications for people who are suffering from the health impacts of air pollution today. It would also be illegal in the face of the High Court’s ruling.”

The government added:

“Air pollution has improved significantly since 2010, but we recognise there is more to do which is why we have put in place a £3.5bn plan to improve air quality and reduce harmful emissions.

“We will also end the sale of conventional new diesel and petrol cars and vans by 2040, and next year we will publish a comprehensive clean air strategy which will set out further steps to tackle air pollution.”